Re: [PATCH v4 6/6] powerpc64/bpf: Additional NVR handling for bpf_throw

[bot+bpf-ci@kernel.org]

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> diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
> index c284a12fce6e..98780a15fc69 100644
> --- a/arch/powerpc/net/bpf_jit_comp64.c
> +++ b/arch/powerpc/net/bpf_jit_comp64.c

[ ... ]

> @@ -125,9 +149,19 @@ int bpf_jit_stack_tailcallinfo_offset(struct codegen_context *ctx)
>
>  static int bpf_jit_stack_offsetof(struct codegen_context *ctx, int reg)
>  {
> -	if (reg >= BPF_PPC_NVR_MIN && reg < 32)
> +	int min_valid_nvreg = BPF_PPC_NVR_MIN;
> +	/* Default frame size for all cases except exception boundary */
> +	int frame_nvr_size = BPF_PPC_STACKFRAME;
> +
> +	/* Consider all nv regs for handling exceptions */
> +	if (ctx->exception_boundary || ctx->exception_cb) {
> +		min_valid_nvreg = _R14;
> +		frame_nvr_size = BPF_PPC_EXC_STACKFRAME;
> +	}
> +
> +	if (reg >= min_valid_nvreg && reg < 32)
>  		return (bpf_has_stack_frame(ctx) ?
> -			(BPF_PPC_STACKFRAME + ctx->stack_size) : 0)
> +			(frame_nvr_size + ctx->stack_size) : 0)
>  				- (8 * (32 - reg)) - BPF_PPC_TAILCALL;

For the redzone case (no stack frame) with exception_boundary, this
formula computes R23 at offset -80 and R24 at offset -72 from R1.

However, bpf_stf_barrier() uses hardcoded offsets -80(1) and -72(1) to
temporarily save R21 and R22 during STF barrier execution:

    asm (
    "	bpf_stf_barrier:		;"
    "		std	21,-80(1)	;"
    "		std	22,-72(1)	;"
    "		sync			;"
    "		ld	21,-80(1)	;"
    "		ld	22,-72(1)	;"
    ...
    );

When an exception_boundary program without its own stack frame executes a
memory load that triggers STF_BARRIER_FALLBACK, can bpf_stf_barrier()
corrupt the saved R23 and R24 values? The hardcoded offsets would overlap
with the extended NVR save area.

If exception_cb later restores these registers, it would get R21/R22 values
instead of the original R23/R24 values.

[ ... ]


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/21265643939